1. Field of Invention
The present invention relates generally to asymmetrically stress biased piezoelectric and electrostrictive devices having integral electrodes and, more particularly, to the use of these devices in a ground fault interrupter circuit.
2. Description of the Prior Art
Electricity is the most prevalent source of energy used throughout the industrialized world, today. Problems, do however, arise if a person comes into contact with an energized conductor and his body becomes a short circuit to ground. Currents of less than 15 milliamps have been known to interrupt the electrical impulses controlling the heart muscle, and cause the heart to fibrillate, resulting in death.
Ground fault interrupter circuits, "GFICs", protect people from the harmful effects of short circuits by detecting the difference between the current delivered to, and the current returning from, a resistive load. When the amount of current returning on the neutral or grounded conductor is less than the current delivered to the load a ground fault condition occurs. If a GFIC is between the ground fault condition and the supply, the GFIC will open the circuit and prevent injury to anyone or anything that has become a path to ground.
In prior GFICs solenoids are typically used to open the circuit when a ground fault condition is present. A core, which is connected to a solenoid, measures the difference between the supply and returning currents. In a non-ground fault condition, (i.e. normal operating condition), the difference between the supply and return currents is zero. Therefore, the magnitude of the magnetic flux created by the supply current cancels out the magnitude of the magnetic flux created by the return current, resulting in no voltage being induced on the core. When a ground fault condition occurs, the supply current is greater than the return current. Consequently, the magnitude of the magnetic flux created by the supply current exceeds that of the magnetic flux created by the return current and a voltage is induced on the core. The induced core voltage energizes the solenoid. The energized solenoid produces a magnetic field which disengages the solenoid's plunger and opens the supply conductor, thus cutting the supply voltage to the ground fault condition.
Problems may arise when magnetic sensitive equipment is used near prior solenoid-actuated GFICs when they are tripped. Even though the magnetic fields produced within the core and the solenoid expand and collapse almost instantaneously, they are powerful and may damage memory storage devices such as "floppy" disks. Problems may also occur in prior GFICs if dust shorts the solenoid's windings, resulting in an insufficient voltage to disengage the plunger. Dust may also prevent the plunger from fully disengaging, thus rendering the GFIC ineffective.
Another typical problem associated with solenoid type GFICs is their inability to be used near equipment, such as transformers, which produce powerful magnetic fields. Powerful magnetic fields can cause the plunger to be inadvertently disengaged, thus cutting the supply voltage when a ground fault condition does not exist.